Method for producing granular iron

ABSTRACT

A method for producing granular iron comprising: charging agglomerates formed from a raw material mixture containing an iron oxide-containing substance and a carbonaceous reducing agent onto a carbonaceous material spread on a hearth of a furnace; and heating the agglomerates to thereby reduce and melt iron oxides in the agglomerates, wherein the temperature of the agglomerates in the furnace is set in a range between 1200° C. and 1500° C.; the oxygen partial pressure in atmospheric gas under which the agglomerates are heated is set to 2.0×10 −13  atm or more at standard state; and the linear speed of the atmospheric gas in the furnace is set to 4.5 cm/second or more.

TECHNICAL FIELD

This invention relates to a method for producing granular iron by: charging agglomerates formed from a raw material mixture, which contains an iron oxide-containing substance and a carbonaceous reducing agent, onto a carbonaceous material spread on a hearth of a furnace; and heating the agglomerates to thereby reduce and melt iron oxides in the agglomerates.

BACKGROUND ART

The direct reduced iron producing method has been developed for making granular metallic iron from a raw material mixture which contains an iron oxide source such as iron ore or iron oxide (hereinafter referred to as iron oxide-containing substance) and a carbonaceous reducing agent. With this producing method, the granular metallic iron is made by: charging the raw material mixture onto a hearth of a heating furnace; heating the raw material mixture with the heat transferred by gas from burners in the furnace or radiant heat to thereby reduce iron oxides in the raw material mixture by the carbonaceous reducing agent into reduced iron, carburize and melt the reduced iron, followed by coalesce it to granules while separating it from subgenerated slag; and then cooling and solidifying it. This producing method has been the subject of considerable practical research of late, because it requires no large-scale facility such as a blast furnace, and because it affords greater flexibility in terms of resources such as not requiring coke. However, the producing method has various problems to be solved in order to be applied on an industrial scale, including stability of operation, safety, cost, quality of the granular iron (product), and so forth.

Since the granular iron made by the above-mentioned direct reduced iron producing method is sent to an existing steelmaking facility (such as an electric furnace or a converter) and is used as an iron source, it preferably has a low content of impurity elements. The carbon content of the granular iron is preferably as high as possible, without excessive range, in order to increase its applicability as an iron source.

In an effort to improve the quality of granular iron, the present applicant has proposed a granular iron having a high Fe purity of 94 mass % or more and a carbon content adjusted to between 1.0 and 4.5 mass % in Japanese Unexamined Patent Application Publication No. 2002-339009 (Patent Document 1). This granular iron is further adjusted to have a sulfur content of 0.20 mass % or less, a silicon content of 0.02 to 0.5 mass and a manganese content of less than 0.3 mass %. However, adjusting the phosphorus content of the granular iron is not disclosed in Patent Document 1. The reason for this is as follows: since the behavior of phosphorus in the reduction process of iron oxide is already clear from the chemical reaction mechanism in blast furnace, it is recognized that almost all phosphorus sourced from a material to be reduced (that is, raw material) remains in a reduced product (that is, metallic iron) under reductive atmosphere and that the phosphorus does not move into subgenerated slag, and thus it is also recognized that the phosphorus content in the raw material has to be decreased, and/or that the granular iron made by the producing method disclosed in Patent Document 1 has to be subjected to a further dephosphorization, in order to reduce the phosphorus content of the granular iron.

In recent years, the grade of iron ore has tended to be on the decline, and the amount of phosphorus contained in mined iron ore is on the rise. Therefore, in the future it will be increasingly difficult to procure raw materials with a low phosphorus content. However, a further dephosphorization subjected to the granular iron made by the producing method disclosed in Patent Document 1 for reducing its phosphorus content leads to higher cost.

DISCLOSURE OF THE INVENTION

The present invention is developed based on the above-mentioned background, and has an object to provide a method for producing granular iron having a low phosphorus content by: charging agglomerates formed from a raw material mixture containing an iron oxide-containing substance and a carbonaceous reducing agent onto a carbonaceous material spread on a hearth of a furnace; and heating the agglomerates to thereby reduce and melt iron oxides in the agglomerates.

One aspect of the present invention is directed to a method for producing granular iron comprising: charging agglomerates formed from a raw material mixture containing an iron oxide-containing substance and a carbonaceous reducing agent onto a carbonaceous material spread on a hearth of a furnace; and heating the agglomerates to thereby reduce and melt iron oxides in the agglomerates, wherein the temperature of the agglomerates in the furnace is set in a range between 1200° C. and 1500° C.; the oxygen partial pressure in atmospheric gas under which the agglomerates are heated is set to 2.0×10¹³ atm or more at standard state; and the linear speed of the atmospheric gas in the furnace is set to 4.5 cm/second or more.

The object, features, aspects and advantages of the present invention will become clearer through reference to the following detailed description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the relation between the dephosphorization ratio and the gas linear speed under different oxygen partial pressures.

FIG. 2 is a graph showing the relation between the dephosphorization ratio and the gas linear speed.

FIG. 3 is a graph showing the relation between the dephosphorization ratio and the oxygen partial pressure.

FIG. 4 is a graph showing the relation between the dephosphorization ratio and the discharging time.

FIG. 5 is a graph showing the relation between the dephosphorization ratio and the amount of fixed carbon contained in the carbonaceous reducing agent blended to the raw material mixture.

BEST MODE FOR CARRYING OUT THE INVENTION

The metallurgical process of producing granular iron by: charging agglomerates formed from a raw material mixture containing an iron oxide-containing substance and a carbonaceous reducing agent onto a carbonaceous material spread on a hearth of a furnace; and heating the agglomerates to thereby reduce and melt iron oxides in the agglomerates is usually carried out under reductive atmosphere. The reason for this is as follows: when the process is performed under oxidative atmosphere, the reduction of the iron oxides contained in the agglomerates comes to a standstill during the heating of the agglomerates, and reduced iron cannot be obtained at a high yield. On the other hand, when the process is performed under reductive atmosphere, reduction of the iron oxides proceeds well. However, almost none of the phosphorus contained in the reduced iron move to slag subgenerated in the reduction, and still remain in the granular iron made by melting the reduced iron due to this melting under reductive atmosphere. As a result, granular iron with a high phosphorus content is obtained. To lower the phosphorus content of granular iron, the obtained granular iron is required to be supplied to an electric furnace, for example, and subjected to a further dephosphorization.

During reducing and melting the above-mentioned agglomerates at a high temperature between 1200 and 1500° C., the carbonaceous reducing agent causes reductive gas to spew out of the interior of the agglomerates while the iron oxides in the agglomerates are being reduced, in contrast, almost no reductive gas is generated after the reduction of the iron oxides are almost over and the reduced iron melts and separates to granular iron and subgenerated slag. Accordingly, the inventors considered that the composition of the granular iron during the period in which the reduced iron melts and separates to granular iron and subgenerated slag is greatly influenced by the composition of its atmospheric gas. In view of this, the inventors conducted diligent study in the vision that the suitable control of the atmospheric gas under which the reduced iron melts and separates to granular iron and subgenerated slag can adjust the composition of the granular iron. As a result, the inventors found that

(I) charging the agglomerates onto a carbonaceous material spread on a hearth of a furnace and then heating the agglomerate of which temperature is to be in a range between 1200° C. and 1500° C.,

(II) setting the oxygen partial pressure in atmospheric gas under which the agglomerates are heated to 2.0×10⁻¹³ atm or more at standard state, and

(III) setting the linear speed of the atmospheric gas in the furnace to 4.5 cm/second or more make the phosphorus contained in the reduced iron move to slag subgenerated in the reduction while the reduced iron is melting. The inventors also found that granular iron with a low phosphorus content can be produced based on this knowledge, and thus accomplished the present invention.

The present invention will now be described in terms of the procedure of producing granular iron.

(I) Agglomerates are formed by agglomerating a raw material mixture that contains a carbonaceous reducing agent and an iron oxide-containing substance.

The above-mentioned iron oxide-containing substance can be, for example, iron ore, iron sand, nonferrous smelting slag, and so forth. The above-mentioned carbonaceous reducing agent can be, for example, a carbon-containing substance, more specifically, coal, coke, or the like.

Other components can also be added to the above-mentioned raw material mixture, such as a binder, an MgO supplying substance, or a CaO supplying substance. Binders can be, for example, polysaccharides (such as wheat gluten and other starches). MgO supplying substances can be, for example, MgO powder, magnesium-containing substances extracted from natural ore, seawater, or the like, and magnesium carbonate (MgCO₃). CaO supplying substances can be, for example, burnt lime (CaO) and limestone (whose main component is CaCO₃).

There are no particular limitations on the shape of the agglomerates. For example, it can be in the form of a pellet or a briquette. Nor are there any particular limitations on the size of the agglomerates. In terms of operations, the agglomerate size (maximum diameter) is preferably 50 mm or smaller, and it is preferably about 5 mm or larger. This is because too large agglomerate size decreases heat transfer to the lower part of the pellet and results in poor productivity, additionally decreasing agglomerating efficiency. Therefore, the agglomerate size is preferably 50 mm or less.

Carbonaceous material is spread on the hearth in advance in order to reduce the agglomerates. This is because the carbonaceous material serves as a carbon supply source when not enough carbon is contained in the agglomerates, and also acts to protect the hearth.

It is recommended that the carbonaceous material that is spread on the hearth have a maximum particle size of 2 mm or less. Using a carbonaceous material with a maximum particle size of 2 mm or less, can suppress that molten slag runs down into the gaps in the carbonaceous material. As a result, this prevents the molten slag from reaching the surface of the hearth and corroding the hearth. The lower limit to the maximum particle size of the carbonaceous material is preferably about 0.5 mm, for example. Using a carbonaceous material in which lower limit to the maximum particle size is about 0.5 mm can suppress that the agglomerates sink into the carbonaceous material layer. As a result, this prevents a drop in heating rate and a decrease in productivity. Spreading the carbonaceous material on the hearth is preferably in a thickness of about 1 to 5 mm, for example.

Then, the agglomerates that have been prepared are charged to a hearth on which the carbonaceous material spreads and heated so that the temperature of the agglomerates becomes between 1200 and 1500° C., thereby the iron oxides in the raw material mixture are reduced and melted. The temperature of the agglomerates is preferably 1250° C. or more. Setting the temperature to 1250° C. or more shortens the melting time of the granular iron and slag, and also accelerates the separation of the slag from the granular iron, allowing granular iron with a higher iron purity to be obtained. On the other hand, the temperature of the agglomerates is preferably 1450° C. or less. Setting the temperature to 1450° C. or less does not require the heating furnace to be a complicated structure, and can suppress a decrease in thermal efficiency. From the standpoints of the heating furnace structure and energy use, the targeted metallic iron nuggets are preferably produced at lower temperature. When burners are used as the heating means in the furnace, the temperature of the agglomerates can be adjusted by controlling the combustion conditions of these burners. There are no particular limitations on the type of furnace used in the present invention, for example, a heating furnace or a moving hearth furnace can be used. A rotary hearth furnace can be used, for example, as a moving hearth furnace.

(II and III) The oxygen partial pressure in atmospheric gas under which the agglomerates are heated is set to 2.0×10⁻¹³ atm or more at standard state, and that the linear speed of this gas is set to 4.5 cm/second or more. As a result of various experiments, the inventors found the followings: when the reduced iron is melted under a slightly oxidative atmosphere, the phosphorus contained in the reduced iron is oxidized, and this phosphorus moves to the slag, and this decreases the phosphorus content of the granular iron. More specifically, when the oxygen partial pressure of the atmospheric gas is less than 2.0×10⁻¹³ atm, or when the gas linear speed is less than 4.5 cm/second, the dephosphorization of the granular iron cannot be accelerated, since not enough oxidative gas is contained in the atmospheric gas near the surface of the agglomerates. Therefore, the oxygen partial pressure of the atmospheric gas under which the agglomerates are heated is set to 2.0×10⁻¹³ atm or more at standard state, and the gas linear speed is set to 4.5 cm/second or more.

The oxygen partial pressure of the atmospheric gas is preferably 2.8×10⁻¹³ atm or more at standard state. The higher is the oxygen partial pressure, the more the dephosphorization of the granular iron is accelerated. However, excessively high oxygen partial pressure causes the granular iron to be re-oxidized, and this decreases the iron purity (metallization ratio). Therefore, the oxygen partial pressure is preferably 4.8×10⁻¹³ atm or less at standard state, and more preferably 4.0×10⁻¹³ atm or less at standard state.

The linear speed of the atmospheric gas in the furnace is preferably 5 cm/second or more. The higher is the gas linear speed, the more the dephosphorization of the granular iron is accelerated. However, excessively high gas linear speed causes the granular iron to be re-oxidized, and this decreases the iron yield. Therefore, the gas linear speed is preferably 13.5 cm/second or less, and more preferably 9 cm/second or less.

The phrase “atmospheric gas under which the agglomerates are heated” means the atmospheric gas near the surface of the agglomerates. The phrase “near the surface of the agglomerates” means the area up to a height of 50 mm from the surface of the agglomerates. Since the oxygen partial pressure and the linear speed of the atmospheric gas in the furnace are often different at the bottom portion of the furnace (near the hearth) and the top portion of the furnace (near the roof), the above-mentioned oxygen partial pressure and gas linear speed are specified for the atmospheric gas near the surface of the agglomerates, which affect the redox reaction of the agglomerates.

The oxygen partial pressure of the atmospheric gas under which the agglomerates are heated can be calculated by taking a sample of the atmospheric gas near the surface of the agglomerates, and analyzing the gas composition. The linear speed of the atmospheric gas can be measured with a pitot tube or the like.

The oxygen partial pressure of the atmospheric gas can be controlled, for example, by: adjusting the amount of oxygen fed to the burners; adjusting the amount of fuel fed to the burners or the air ratio, etc.; or adjusting the injection of reductive gas. The linear speed of the atmospheric gas can be controlled, for example, by: adjusting the amount of gas fed to the burners; adjusting the injection angle of the burners; or varying the roof height.

The oxygen partial pressure and the linear speed of the atmospheric gas are adjusted so as to be within the above-mentioned ranges at latest from the point when the melting of the reduced iron begins. This is because the composition of the granular iron is actually affected by the atmospheric gas composition more during melting than during solid reduction.

Preferably, the linear speed of the atmospheric gas, under which the agglomerates are heated, is controlled to 5.4 cm/second or less (including 0 cm/second) until the iron oxides, which are contained in the raw material mixture, begins to melt; and the linear speed of the atmospheric gas, under which the agglomerates are heated, is controlled to 4.5 cm/second or more, once the iron oxides begins to melt. In the period before the iron oxides begins to melt, the reduction reaction is very actively occurring within the agglomerates, this causes a difficult change in the composition of the atmospheric gas near the surface of the agglomerates or inside the agglomerates, even if the composition of the atmospheric gas in the furnace is changed. Meanwhile, as the solid reduction nears completion, melting of iron begins due to the beginning of carburization in iron and the decrease of the melting point of the resulting iron. When the iron begins to melt, almost no gas is generated from the agglomerates, and thus the composition of the iron is greatly affected by the composition of the atmospheric gas. Therefore, the linear speed of the atmospheric gas under which the agglomerates are heated is preferably controlled to suitable levels up in the period before the iron oxides begins to melt and after the melting begins respectively. Incidentally, the oxygen partial pressure of the atmospheric gas in the period before the iron oxides contained in the raw material mixture begins to melt is preferably 2.8×10⁻¹³ atm or less.

Thus, in the present invention, it is preferable to control the oxygen partial pressure and the linear speed of the atmospheric gas at the time until the iron oxides begins to melt and after the melting begins, for example, when a moving hearth furnace is used as the heating furnace, partitions can be suspended from the furnace roof so that the inside of the furnace is divided into a plurality of zones, and the oxygen partial pressure and the linear speed of the atmospheric gas is controlled for each of these zones.

As discussed above, suitable controlling of the oxygen partial pressure and the linear speed of the atmospheric gas in the period of reducing and melting can proceed the dephosphorization of the granular iron more effectively, and can produce granular iron with a lower phosphorus content than reducing and melting just under reductive atmosphere.

In the present invention, it is preferred to adjust the percentage of the amount of fixed carbon contained in the carbonaceous reducing agent blended to the raw material mixture to be in a range between 98 mass % and 102 mass % with respect to the amount of fixed carbon needed to reduce the iron oxides contained in the iron oxide-containing substance. The reason for this is as follows: when the percentage of the amount of fixed carbon contained in the carbonaceous reducing agent is less than 98 mass % with respect to the amount of fixed carbon needed to reduce the iron oxides, this makes lack of carbon, and leads an inadequate reduction of the iron oxides, even though reductive gas (CO gas) rises up from the carbonaceous material spread on the hearth, as discussed below. The required percentage of the amount of fixed carbon contained in the carbonaceous reducing agent is preferably 98 mass % or more, and more preferably 98.5 mass % or more, with respect to the amount of fixed carbon needed to reduce the iron oxides. However, when the amount of fixed carbon contained in the carbonaceous reducing agent is excessively large, the reductive gas (CO gas) still continues to rise up from the agglomerates by reacting with the atmospheric gas even after the reduction is finished. This decreases the oxygen partial pressure in melting the reduced iron, as discussed below, and thus makes the dephosphorization ratio of the reduced iron be lower. Therefore, the required percentage of the amount of fixed carbon contained in the carbonaceous reducing agent is preferably 102 mass % or less, and more preferably 101 mass % or less, with respect to the amount of fixed carbon needed to reduce the iron oxides.

In the present invention, it is especially recommended that the amount of fixed carbon contained in the carbonaceous reducing agent be adjusted somewhat on the low side with respect to the amount of fixed carbon needed to reduce the iron oxides. The reason for this is as follows: the unreduced portions of the iron oxides are reduced by the carbonaceous material spread on the hearth, since the agglomerates are on the carbonaceous material in the present invention, although an insufficient amount of fixed carbon contained in the carbonaceous reducing agent seems to cause inadequate reduction of the granular iron.

Specifically, the iron oxides (FeO_(x)) contained in the agglomerates are reduced by the carbon (C) contained in the carbonaceous reducing agent and by the carbonaceous material spread on the hearth, according to the reduction reactions in the following formulas (1) and (2), to form granular iron.

FeO_(x) +xCO→Fe+xCO₂  (1)

FeO_(x) +xC→Fe+xCO  (2)

As a result of various experiments, the inventors found that the reduction reaction proceeds in the proportions indicated by the following formula (3) when a moles of the FeO_(x) of Formula (1) react, and when b moles of the FeO_(x) of Formula (2) react. That is, Formula (3) indicates the number of oxygen atoms reduced by one carbon atom. In the reduction of FeO_(x) the inventors estimate that about 38% of the total occurs by direct reduction by carbon (C), and about 72% of the total occurs by indirect reduction by reducing gas (CO gas).

1.0≦1+a/(a+b)≦1.5  (3)

Therefore, even though the amount of carbon, which is calculated as one carbon atom needed to reduce one oxygen atom contained in iron oxide, is slightly on the low side (for example, by blending carbonaceous reducing agent a little less into the raw material mixture), iron oxides in agglomerates are still adequately reduced.

Also, adjusting the amount of fixed carbon contained in the carbonaceous reducing agent on the lower side with respect to the amount of fixed carbon needed to reduce the iron oxides causes to form more iron oxides (FeO) contained in subgenerated slag in the reduction, and drives the dephosphorization reaction faster in melting the reduced iron. Therefore, the percentage of the amount of fixed carbon contained in the carbonaceous reducing agent is more preferably 100 mass % or less with respect to the amount of fixed carbon needed to reduce the iron oxides.

The amount of fixed carbon needed to reduce the iron oxide may be calculated from the composition of the raw material mixture.

The granular iron is required to be carburized so that it contains about 3 mass % carbon in order to decrease the melting point of the granular iron during separating the melted granular iron from the slag. However, when the amount of fixed carbon contained in the carbonaceous reducing agent blended to the raw material mixture is set to be slightly insufficient with respect to the amount of fixed carbon needed to reduce the iron oxides, the granular iron does not contain enough fixed carbon, and this results that the granular iron cannot be melted. Accordingly, spreading a carbonaceous material on the hearth and setting the amount of fixed carbon contained in this carbonaceous material to be in excess over the amount of fixed carbon needed to reduce the iron oxides increase the amount of fixed carbon supplied to the granular iron, and this can make the molten granular iron be separated from the slag.

The percentage of the amount of fixed carbon contained in the carbonaceous material that is spread on the hearth is preferably adjusted to within a range between 2 mass % and 5 mass % with respect to the amount of fixed carbon needed to reduce the iron oxides. There are no particular limitations on the type of carbonaceous material that is spread on the hearth, for example, the carbon-containing material used as the above-mentioned carbonaceous reducing agent can be used.

In the present invention, the above-mentioned agglomerates is also preferred to adjust the composition of the raw material mixture so that the basicity of slag subgenerated in reducing the iron oxides is in a range between 1.0 and 1.6. The reason for this is as follows: when the slag basicity is less than 1.0, the dephosphorization reaction in melting the reduced iron does not proceed and thus the phosphorus content of the granular iron cannot be reduced. Therefore, the basicity is preferably 1.3 or more, and more preferably 1.4 or more. However, too high slag basicity causes too high melting point of the slag, and the resulting slag does not melt when the reduced iron is melted, making it difficult to separate the granular iron from the slag. As a result, the slag ends up being mixed in the granular iron, and this degrades in quality of the granular iron. Therefore, the basicity is preferably 1.6 or less.

The basicity of slag is the value [(CaO)/(SiO₂)] calculated from the CaO content and the SiO₂ content in the slag.

EXAMPLES

The present invention will now be described in further detail through examples. These examples are not intended to limit the present invention, various modifications are possible without departing from the scope of the invention described above and below, and these modifications are included in the technical scope of the present invention.

In the examples, each agglomerates were made from raw material mixtures each containing a carbonaceous reducing agent and an iron oxide-containing substance, and then, each granular irons were made by: charging each agglomerates onto a carbonaceous material spread on a hearth of a heating furnace; and heating the agglomerates to thereby reduce and melt the iron oxides in the agglomerates in a laboratory. The compositions of the agglomerates and the conditions of reduction and melting were varied. Specifically, as follows.

Two kinds of iron oxide-containing substance were used: iron ore (n) with a low phosphorus content; and iron ore (hpb) with a high phosphorus content. Table 1 below shows the compositions of the iron ore (n) and the iron ore (hpb). Two kinds of carbonaceous reducing agent were also used: coal (p) with a low phosphorus content; and coal (b) with a high phosphorus content. Table 2 below shows the compositions of the coal (p) and the coal (b).

The iron ore each shown in Table 1 and the coal each shown in Table 2 were blended with additives, and then, each pelletized agglomerates (test material) with particle sizes of 18 to 20 mm were produced. The blended additives were wheat gluten that was added as a binder, MgO, CaO, etc. Table 3 below shows the blending ratio of each test materials (percentages of weight values).

Table 3 shows the target values for the percentage of the amount of fixed carbon contained in the carbonaceous reducing agent blended to the raw material mixture with respect to the amount of fixed carbon needed to reduce the iron oxides. Table 3 also shows the target values for the basicity of slag subgenerated during reduction.

Table 4 below shows the compositions of the test materials. In Table 4, test material (1) is pellets with a low phosphorus content, and test materials (2) to (5) are pellets with a high phosphorus content.

Each granular irons were produced by: charging each test materials shown in Table 4 into the furnace of which hearth was spread with a carbonaceous material; heating them to thereby reduce and melt the iron oxides in the raw material mixture; and discharging products to a cooling zone at the point when the granular iron and slag had completely separated. The number of samples of each test materials charged into the furnace was 30. 130 g of smokeless coal with a maximum particle size of 2 mm or less was spread over the hearth as a carbonaceous material. Extra carbonaceous material was spread around the edges to protect the hearth.

The test materials charged into the furnace were heated with a heater provided to the furnace, so that the temperature of the test materials would reach 1450° C.

Inside the furnace, the linear speed of the atmospheric gas under which the test materials were heated (the linear speed of the atmospheric gas near the test materials) was controlled to be in a range between 1.35 and 20.27 cm/second, and the oxygen partial pressure of the atmospheric gas under which the test materials were heated (the oxygen partial pressure of the atmospheric gas near the test materials) was controlled to be in a range between 0 and 5.057×10⁻¹³ atm. Tables 5 and 6 below show the gas linear speeds and oxygen partial pressures. The gas linear speeds were the value at standard state.

The gas linear speeds were calculated from the amount of gas supplied and the cross sectional area at the sample placement portion inside the furnace. The oxygen partial pressures were calculated by the following procedure.

The following formula (4) expresses a carbon combustion reaction, and the standard generation free energy ΔF in this reaction is expressed by the following formula (5).

C(graphite)+O₂ (g)=CO₂ (g)  (4)

ΔF=−94640+0.05×T (cal/mol)  (5)

Meanwhile, the standard generation free energy ΔF of this reaction is expressed by the following formula (6), using the partial pressure P_(CO2) of the atmospheric gas accounted for by carbon dioxide gas, and the partial pressure P_(O2) of the atmospheric gas accounted for by oxygen gas.

ΔF=−RT×log(P _(CO2) /P _(O2))  (6)

The absolute temperature of 1450° C. is:

1450(° C.)+273=1723(K),

therefore the relation between the carbon dioxide partial pressure and the oxygen partial pressure in the atmospheric gas at 1450° C. is found as follows from the above formulas (5) and (6).

−94640+0.05×1723=−4.575×1723×log(P _(CO2) /P _(O2))

log(P _(CO2) /P _(O2))=11.995

P _(CO2) /P _(O2)=9.887×10¹¹

Here, the partial pressure of the atmospheric gas accounted for by carbon dioxide gas is measured, for example, when P_(CO2)=0.5 is measured, then

P _(O2)=5.0571×10⁻¹³

is obtained as the partial pressure of the atmospheric gas accounted for by oxygen gas.

Tables 5 and 6 show the compositions of the resulting granular irons, and the compositions of the subgenerated slags when granular irons were produced. Of the compositions of the granular irons shown in Tables 5 and 6, each amount of iron is the value calculated by subtracting the amount of alloy elements and impurities from the total (100 mass %).

In Table 6, No. 30 is the result of discharging the granular iron one minute before the point at which the separation of slag and granular iron was complete, and No. 31 is the result of discharging the granular iron from the furnace after waiting for three minutes from the point at which the separation of slag and granular iron was complete. In Tables 5 and 6, everything other than Nos. 30 and 31 is the result of discharging the granular iron from the furnace at the point when one minute had elapsed from the point at which the separation of slag and granular iron was complete.

The center temperatures of the test materials were measured and found to be: approximately 1300° C. at the point one minute before the separation of slag and granular iron was complete (No. 30); approximately 1400° C. at the point when one minute had elapsed since the completion of the separation of slag and granular iron; and approximately 1450° C. at a point three minutes after the completion of the separation of slag and granular iron (No. 31).

Also, the CO₂ gas proportion near the test materials was substantially constant from the point one minute before the completion of the separation of slag and granular iron, up to the point three minutes after the completion of the separation of slag and granular iron. Meanwhile, some CO gas was noted to rise up from the test material at a point one minute before the completion of the separation of slag and granular iron, but no CO gas was seen to rise up from the test material once the separation of slag and granular iron was partially completed.

The dephosphorization ratio was calculated from the composition of the granular iron and the composition of the test material by the following formula.

Dephosphorization ratio (%)=[1−(amount of phosphorus contained in the resulting granular iron/total amount of iron contained in the resulting granular iron)/(amount of phosphorus contained in test material/total amount of iron contained in test material)]×100

FIG. 1 shows the relation between the dephosphorization ratio and the gas linear speed under different oxygen partial pressures, based on the data in Tables 4 and 5. In FIG. 1, the mark ⋄ indicates the result at an oxygen partial pressure of 0 atm, the mark ▴ indicates the result at an oxygen partial pressure of 1.011×10⁻¹³ atm, the mark x indicates the result at an oxygen partial pressure of 1.517×10⁻¹³ atm, the mark O indicates the result at an oxygen partial pressure of 3.034×10⁻¹³ atm, and the mark ▪ indicates the result at an oxygen partial pressure of 5.057×10⁻¹³ atm.

As is clear from FIG. 1, when the atmospheric gas contains oxygen, the higher is the linear speed of the atmospheric gas under which the test material is heated, the higher is the dephosphorization ratio. For example, with test material 3, which had a gas linear speed of 5.41/sec, it can be seen that the dephosphorization ratio rises when the oxygen partial pressure of the atmospheric gas is increased from 1.517×10⁻¹³ atm to 3.034×10⁻¹³ atm, and that with a given test material and at a given gas linear speed, the dephosphorization ratio rises when the oxygen partial pressure of the atmospheric gas is increased. When the oxygen partial pressure of the atmospheric gas is 0 atm (that is, under a nitrogen gas atmosphere), the dephosphorization is not affected by the gas linear speed. When the gas linear speed is less than cm/second, the result for dephosphorization ratio is reversed from that when the oxygen partial pressure of the atmospheric gas is 1.517×10⁻¹³ atm, but these are considered to be affected by sample variance or phosphorus analysis error.

Based on the above results, increasing the gas linear speed and the oxygen partial pressure of the atmospheric gas to the specified values or higher is an effective way to raise the dephosphorization ratio.

FIG. 2 shows the relation between the dephosphorization ratio and the gas linear speed in Nos. 24, 25 and 32, out of the results in Table 6 when the oxygen partial pressure was 3.034×10⁻¹³ atm. A comparison of FIG. 2 with FIG. 1 reveals that even though the amount of phosphorus contained in the test material changes, at the condition that the oxygen partial pressure is constant, the dephosphorization ratio rises along with the gas linear speed. Although not shown in the drawings, the same thing can be understood from Nos. 2, 4 and 6, for example.

FIG. 3 shows the relation between the dephosphorization ratio and the oxygen partial pressure in Nos. 25, 27, 28 and 29, out of the results in Table 6 when the gas linear speed was 5.41 cm/second. As is clear from FIG. 3, when the gas linear speed is constant, the dephosphorization ratio rises along with the oxygen partial pressure. Also, when the oxygen partial pressure is 1.517×10⁻¹³ atm, it can be seen that there is almost no change in the dephosphorization ratio. Although not shown in the drawings, it can be understood from Nos. 3, 4 and 5, for example, that when the gas linear speed is constant, the dephosphorization ratio rises along with the oxygen partial pressure.

FIG. 4 shows the relation between the dephosphorization ratio and the time discharging granular iron in Nos. 25, 30 and 31, out of the results in Table 6 when the oxygen partial pressure was 3.034×10⁻¹³ atm and the gas linear speed was 5.41 cm/second. FIG. 4 shows the dephosphorization ratio of variation compared with the variation of the time discharging the granular iron which was separated from the slag out of the furnace from the clock time when the slag and granular iron had been completely separated was set 0 minute, after the reduced irons were melted. As is clear from FIG. 4, the dephosphorization ratio drops when heating is continued after the slag and granular iron have been separated.

The highest dephosphorization ratio in FIG. 4 is when the discharging time is “−1 minute,” and this “−1 minute” means that the granular iron was discharged from the furnace before the granular and slag were separated, which is a condition that cannot be employed in actual practice.

FIG. 5 shows the relation between the dephosphorization ratio and the amount of fixed carbon contained in the carbonaceous reducing agent blended to the raw material mixture for Nos. 21, 22 and 25, out of the results shown in Table 6. As is clear from FIG. 5, the dephosphorization ratio is advantageously higher when the amount of fixed carbon contained in the carbonaceous reducing agent blended to the raw material mixture is set on the low side with respect to the amount of fixed carbon needed to reduce the iron oxides.

On the other hand, it can be seen that there is a further drop in the dephosphorization ratio when the percentage of the amount of fixed carbon contained in the carbonaceous reducing agent blended to the raw material mixture is over 102 mass % with respect to the amount of fixed carbon needed to reduce the iron oxides. This is considered to be attributable to the fact that since a large amount of reductive gas rises up even in the process of melting the reduced iron, the effect of increasing the gas linear speed is lost.

As is clear from the results for No. 22, even though the amount of fixed carbon contained in the carbonaceous reducing agent blended to the raw material mixture is set on the low side with respect to the amount of fixed carbon needed to reduce the iron oxides contained in the test material, adjusting the amount of carbon contained in the carbonaceous material spread on the hearth to be in a range between 2 and 5 mass % with respect to the amount of fixed carbon needed to reduce the iron oxides causes stable reduction of the iron oxides remaining after dephosphorization has proceeded by the carbonaceous material spread on the hearth.

TABLE 1 COMPOSITION (mass %) IRON TOTAL ORE AMOUNT TYPE OF IRON FeO SiO₂ CaO Al₂O₃ MgO S P (n) 67.64 29.13 4.85 0.44 0.23 0.47 0.004 0.018 (hpb) 62.97 0.47 3.25 0.04 2.08 0.04 0.030 0.13

TABLE 2 COMPOSITION (mass %) TOTAL TOTAL TOTAL AMOUNT VOLATILE ASH FIXED AMOUNT AMOUNT OF PHOSPHORUS COAL TYPE CONTENT CONTENT CARBON OF SULFUR OF CARBON (CALCULATED VALUE) (p) 16.79 4.64 78.57 0.595 86.46 0.00243 COMPOSITION OF ASH CONTENT (mass %) Fe₂O₃ SiO₂ CaO Al₂O₃ MgO TiO₂ P₂O₅ 15.75 43.55 3.98 27.03 1.85 1.67 0.12 COMPOSITION (mass %) TOTAL TOTAL TOTAL AMOUNT VOLATILE ASH FIXED AMOUNT AMOUNT OF PHOSPHORUS COAL TYPE CONTENT CONTENT CARBON OF SULFUR OF CARBON (CALCULATED VALUE) (b) 14.03 4.75 81.22 0.458 83.31 0.03254 COMPOSITION OF ASH CONTENT (mass %) Fe₂O₃ SiO₂ CaO Al₂O₃ MgO TiO₂ P₂O₅ 4.26 57.25 2.16 23.93 0.59 0.87 1.57

TABLE 4 COMPOSITION (mass %) TOTAL TOTAL TEST AMOUNT OF MOUNT OF MATERIAL CARBON IRON SiO₂ Al₂O₃ CaO MgO F P S (1) 15.62 48.60 4.01 0.48 5.96 0.97 0.35 0.017 0.104 (2) 15.90 46.01 2.92 1.67 4.17 0.87 0.36 0.100 0.094 (3) 15.89 46.63 3.31 1.75 4.11 0.80 0.32 0.099 0.094 (4) 14.32 47.29 2.98 1.72 4.20 0.87 0.36 0.100 0.087 (5) 16.14 46.55 3.41 1.72 4.31 0.84 0.36 0.094 0.133

TABLE 5 OXYGEN DEPHOSPHORI- GAS LINEAR PARTIAL COMPOSITION OF COMPOSITION ZATION TEST SPEED PRESSURE × GRANULAR IRON (mass %) OF SLAG (mass %) RATIO No. MATERIAL (cm/sec) 10⁻¹³(atm) C P S Fe FeO P S BASICITY (%) 1 (1) 20.27 5.057 2.9 0.005 0.134 96.92 4.96 0.110 0.244 1.47 85.25 2 (1) 20.27 3.034 3.04 0.007 0.12 96.77 2.84 0.099 0.313 1.53 79.32 3 (1) 9.01 5.057 3.18 0.014 0.114 96.64 1.9 0.068 0.379 1.52 58.59 4 (1) 9.01 3.034 3 0.018 0.091 96.83 0.98 0.037 0.486 1.43 46.86 5 (1) 9.01 1.011 3.49 0.027 0.038 96.39 0.27 0.011 0.712 1.54 19.92 6 (1) 5.41 3.034 3.23 0.023 0.056 96.63 0.45 0.020 0.589 1.43 31.95 7 (1) 1.35 1.011 3.64 0.031 0.028 96.21 0.24 0.007 0.762 1.57 7.89 8 (1) 20.27 5.057 2.15 0.005 0.164 97.65 6.83 0.086 0.292 1.41 85.36 9 (1) 20.27 3.034 2.53 0.005 0.155 97.28 3.5 0.090 0.317 1.47 85.31 10 (1) 9.01 3.034 2.65 0.019 0.111 97.19 0.72 0.032 0.501 1.44 44.11 11 (1) 5.41 3.034 2.97 0.024 0.079 96.89 0.43 0.014 0.668 1.42 29.18

TABLE 6 OXYGEN DEPHOSPHORI- GAS LINEAR PARTIAL COMPOSITION OF COMPOSITION ZATION TEST SPEED PRESSURE × GRANULAR IRON (mass %) OF SLAG (mass %) RATIO No. MATERIAL (cm/sec) 10⁻¹³(atm) C P S Fe FeO P S BASICITY (%) 21 (2) 5.41 3.034 3.00 0.16 0.089 96.72 0.17 0.180 0.336 1.52 23.89 22 (4) 5.41 3.034 2.90 0.07 0.121 96.88 3.94 0.640 0.198 1.50 64.85 23 (5) 5.41 3.034 2.66 0.14 0.110 97.06 0.46 0.240 0.453 1.62 28.57 24 (3) 1.35 3.034 2.96 0.16 0.073 96.78 0.47 0.136 0.378 1.47 22.13 25 (3) 5.41 3.034 2.87 0.14 0.113 96.85 0.42 0.150 0.514 1.61 31.91 26 (3) 20.27 3.034 2.91 0.15 0.103 96.81 0.81 0.250 0.292 0.68 27.02 27 (3) 5.41 1.517 2.62 0.16 0.081 97.11 0.54 0.181 0.367 1.48 22.39 28 (3) 5.41 0 2.78 0.16 0.063 96.95 0.44 0.110 0.487 1.49 22.27 29 (3) 5.41 5.057 2.79 0.11 0.109 96.96 2.16 0.390 0.225 1.48 46.56 30 (3) 5.41 3.034 1.79 0.13 0.091 97.86 1.41 0.300 0.312 1.49 37.43 31 (3) 5.41 3.034 3.29 0.15 0.111 96.42 0.80 0.200 0.237 1.47 26.72 32 (3) 9.01 3.034 2.69 0.11 0.113 97.06 2.82 0.510 0.223 1.48 46.62 33 (5) 5.41 3.034 2.82 0.14 0.113 96.88 0.48 0.140 0.554 1.61 28.44

As described above, one aspect of the present invention is a method for producing granular iron comprising: charging agglomerates formed from a raw material mixture containing an iron oxide-containing substance and a carbonaceous reducing agent onto a carbonaceous material spread on a hearth of a furnace; and heating the agglomerates to thereby reduce and melt iron oxides in the agglomerates, wherein the temperature of the agglomerates in the furnace is set in a range between 1200° C. and 1500° C.; the oxygen partial pressure in atmospheric gas under which the agglomerates are heated is set to 2.0×10⁻¹³ atm or more at standard state; and the linear speed of the atmospheric gas in the furnace is set to 4.5 cm/second or more.

According to the present invention, since the reduced agglomerates are melted in a state in which the oxygen partial pressure of the atmospheric gas and the gas linear speed and are controlled to the above-mentioned conditions, the phosphorus contained in the reduced iron can be moved to the subgenerated slag during reduction. As a result, the granular iron made by melting the reduced iron contains less phosphorus.

In the above-mentioned method for producing granular iron, it is preferable that the composition of the raw material mixture is adjusted so that the percentage of the amount of fixed carbon contained in the carbonaceous reducing agent is in a range between 98 mass % and 102 mass % with respect to the amount of fixed carbon needed to reduce the iron oxides. This causes the reduction of iron oxides to proceed more actively and the granular iron with lower phosphorus content to be produced.

In the above-mentioned method for producing granular iron, it is preferable that the composition of the raw material mixture is adjusted so that the basicity of slag subgenerated in reducing the iron oxides is in a range between 1.0 and 1.6. This causes the dephosphorization reaction to proceed faster and the granular iron with lower phosphorus content to be produced.

In the above-mentioned method for producing granular iron, it is preferable that the percentage of the amount of fixed carbon contained in the carbonaceous reducing agent is in a range between 98 mass % and 100 mass % with respect to the amount of fixed carbon needed to reduce the iron oxides. This causes the amount of fixed carbon contained in the carbonaceous reducing agent to be on the low side with respect to the amount of fixed carbon needed to reduce the iron oxides, and thus more iron oxides (FeO) contained in the subgenerated slag during reduction to be produced. As a result, this accelerates the dephosphorization reaction during the melting of the reduced iron, therefore, the dephosphorization ratio of the reduced iron can be further increased.

In the above-mentioned method for producing granular iron, it is preferable that the linear speed of the atmospheric gas is set to 5.4 cm/second or less (including 0 cm/second) until the iron oxides begins to melt; and the linear speed of the atmospheric gas is set to 4.5 cm/second or more after the iron oxides begins to melt. Adjusting the linear speed of the atmospheric gas under which the agglomerates are heated, both until the iron oxides begins to melt and after the melting has begun, allows the reduction reaction to proceed actively in the agglomerates up until the melting of the iron oxide begins, and allows the melting of the iron to proceed stably after melting has begun.

In the above-mentioned method for producing granular iron, it is preferable that the percentage of the amount of fixed carbon contained in the carbonaceous material which is spread on the hearth is set in a range between 2 mass % and 5 mass % with respect to the amount of fixed carbon needed to reduce the iron oxides; and the maximum particle size of the carbonaceous material is set to 2 mm or less. This increases the amount of fixed carbon supplied to the granular iron, allows the molten granular iron to separate from slag, and also prevents the molten slag from running down into the crevices in the carbonaceous material and corroding the hearth.

INDUSTRIAL APPLICABILITY

Granular iron with a low phosphorus content can be made stably by using the method of the present invention for producing granular iron. 

1. A method for producing granular iron, comprising: charging an agglomerate formed from a raw material mixture comprising an iron oxide comprising substance and a carbonaceous reducing agent onto a carbonaceous material spread on a hearth of a furnace; and heating the agglomerate to thereby reduce and melt at least one iron oxide in the agglomerate, wherein a temperature of the agglomerate in the furnace is in a range between 1200° C. and 1500° C., an oxygen partial pressure in atmospheric gas under which the agglomerate is heated is to 2.0×10⁻¹³ atm or more at standard state, and a linear speed of the atmospheric gas in the furnace is 4.5 cm/second or more.
 2. The method of claim 1, wherein a composition of the raw material mixture is adjusted so that a percentage of an amount of fixed carbon comprised in the carbonaceous reducing agent is in a range between 98 mass % and 102 mass % with respect to an amount of fixed carbon needed to reduce the at least one iron oxide.
 3. The method of claim 1, wherein a composition of the raw material mixture is adjusted so that a basicity of slag subgenerated in reducing the at least one iron oxide is in a range between 1.0 and 1.6.
 4. The method of claim 1, wherein a percentage of an amount of fixed carbon comprised in the carbonaceous reducing agent is in a range between 98 mass % and 100 mass % with respect to an amount of fixed carbon needed to reduce the at least one iron oxide.
 5. The method of claim 1, wherein the linear speed of the atmospheric gas is 5.4 cm/second or less (including 0 cm/second) until the at least one iron oxide begins to melt, and the linear speed of the atmospheric gas is 4.5 cm/second or more after the at least one iron oxide begins to melt.
 6. The method of claim 1, wherein a percentage of an amount of fixed carbon comprised in the carbonaceous material which is spread on the hearth is in a range between 2 mass % and 5 mass % with respect to an amount of fixed carbon needed to reduce the at least one iron oxide, and a maximum particle size of the carbonaceous material is 2 mm or less.
 7. The method of claim 1, wherein the agglomerate size (maximum diameter) is 50 mm or smaller.
 8. The method of claim 1, wherein the agglomerate size (maximum diameter) is 5 mm or larger.
 9. The method of claim 7, wherein the agglomerate size (maximum diameter) is 5 mm or larger.
 10. The method of claim 1, wherein the temperature of the agglomerate in the furnace is in a range between 1250° C. and 1500° C.
 11. The method of claim 1, wherein the temperature of the agglomerate in the furnace is in a range between 1200° C. and 1450° C.
 12. The method of claim 1, wherein the temperature of the agglomerate in the furnace is in a range between 1250° C. and 1450° C.
 13. The method of claim 1, wherein the oxygen partial pressure of the atmospheric gas under which the agglomerate is heated is 2.8×10⁻¹³ atm or more at standard state.
 14. The method of claim 1, wherein the oxygen partial pressure of the atmospheric gas under which the agglomerate is heated is 4.8×10⁻¹³ atm or less at standard state.
 15. The method of claim 1, wherein the oxygen partial pressure of the atmospheric gas under which the agglomerate is heated is 4.0×10⁻¹³ atm or less at standard state.
 16. The method of claim 13, wherein the oxygen partial pressure of the atmospheric gas under which the agglomerate is heated is 4.8×10⁻¹³ atm or less at standard state.
 17. The method of claim 13, wherein the oxygen partial pressure of the atmospheric gas under which the agglomerate is heated is 4.0×10⁻¹³ atm or less at standard state.
 18. The method of claim 1, wherein the linear speed of the atmospheric gas in the furnace is 5 cm/second or more.
 19. The method of claim 1, wherein the linear speed of the atmospheric gas in the furnace is 13.5 cm/second or less.
 20. The method of claim 1, wherein the linear speed of the atmospheric gas in the furnace is 9 cm/second or less. 